Light olefins, defined herein as ethylene and propylene, serve as feeds for the production of numerous important chemicals and polymers. Light olefins traditionally are produced by cracking petroleum feeds. Because of the limited supply and escalating cost of petroleum feeds, the cost of producing olefins from petroleum sources has increased steadily. Efforts to develop and improve olefin production technologies, particularly light olefins production technologies, have increased.
The petrochemical industry has known for some time that oxygenates, especially alcohols, are convertible into light olefin(s). There are numerous technologies available for producing oxygenates including fermentation or reaction of synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials including coal, recycled plastics, municipal waste or any other organic material. Generally, the production of synthesis gas involves a combustion reaction of natural gas, mostly methane, and an oxygen source into hydrogen, carbon monoxide and/or carbon dioxide. Syngas production processes are well known, and include conventional steam reforming, autothermal reforming, or a combination thereof.
Methanol, the preferred alcohol for light olefin production, is typically synthesized from the catalytic reaction of hydrogen, carbon monoxide and/or carbon dioxide in a methanol reactor in the presence of a heterogeneous catalyst. For example, in one synthesis process methanol is produced using a copper/zinc oxide catalyst in a water-cooled tubular methanol reactor. The preferred methanol conversion process is generally referred to as a methanol-to-olefin(s) process, where methanol is converted to primarily ethylene and/or propylene in the presence of a molecular sieve.
In an oxygenate to olefin (OTO) reaction system, a feedstock containing an oxygenate is vaporized and introduced into a reactor. Exemplary oxygenates include alcohols such as methanol and ethanol, dimethyl ether, methyl ethyl ether, methyl formate, and dimethyl carbonate. In a methanol to olefin (MTO) reaction system, the oxygenate-containing feedstock includes methanol. In the reactor, the methanol contacts a catalyst under conditions effective to create desirable light olefins. Typically, molecular sieve catalysts have been used to convert oxygenate compounds to olefins. Silicoaluminophosphate (SAPO) molecular sieve catalysts are particularly desirable in such conversion processes because they are highly selective in the formation of ethylene and propylene.
In a typical MTO reactor system, undesirable byproducts may be formed through side reactions. For example, the metals in conventional reactor walls may act as catalysts in one or more side reactions. If the methanol contacts the metal reactor wall at sufficient temperature and pressure, the methanol may be converted to undesirable methane and/or other byproducts.
Byproduct formation in an MTO reactor is undesirable for several reasons. First, increased investment is required to separate and recover the byproducts from the desired light olefins. Additionally, as more byproducts are formed, less light olefins are synthesized. In other words, the production of byproducts is undesirable because methanol feed is consumed to produce the byproducts. Further, although the relative concentrations of metal catalyzed side reaction byproducts are generally quite low, the total amount of byproducts produced on an industrial scale can be enormous. Thus, it is desirable to decrease or eliminate the synthesis of byproducts in an MTO reactor system.
Sulfur-containing chemicals have proven effective for deactivating or passivating the metal surface of a reactor thereby reducing the formation of undesirable byproducts in the reactor. For example, Japanese Laid Open Patent Application JP 01090136 to Yoshinari et al. is directed to a method for preventing decomposition of methanol or dimethyl ether and coking by sulfiding the metal surface of a reactor. More particularly, the method includes reacting methanol and/or dimethyl ether in the presence of a catalyst at above 450° C. in a tubular reactor made of Iron and/or Nickel or stainless steel. The inside wall of the reactor is sulfided with a compound such as carbon disulfide, hydrogen disulfide or dimethyl sulfide. Additionally, a sulphur compound may be added to the feed.
Although passivating chemicals have proven effective in reducing metal catalyzed side reactions, the introduction of deactivating or passivating chemicals increases production costs because these chemicals or their products must be separated from the desired product. Thus, a need exists for a method and apparatus for reducing the formation of metal catalyzed side reaction byproducts in an MTO reactor system while minimizing the use of deactivating or passivating chemicals.